1. Field of the Invention
The present invention relates to a gas sensor control apparatus for controlling a gas sensor including a gas sensor element adapted to detect the concentration of NOx gas, ammonia gas, or a like gas, and more particularly, to a gas sensor control apparatus which performs heater control processing for activating the gas sensor element through application of heat by a heater.
2. Description of Related Art
A conventionally known gas sensor element has a first measurement chamber, a first oxygen pump cell, a second measurement chamber, a second oxygen pump cell, a reference-oxygen chamber, and an oxygen-concentration detection cell (refer to, for example, Japanese Patent Application Laid-Open (Kokai) No. 10-288595). In the gas sensor element, each of the first oxygen pump cell and the oxygen-concentration detection cell is configured such that an oxygen-ion-conductive solid electrolyte layer has porous electrodes provided on respective opposite sides of the cell. Also, the first measurement chamber communicates with an object gas (an “object gas of measurement”) atmosphere via a first diffusion control layer, and the second oxygen pump cell is configured such that an oxygen-ion-conductive solid electrolyte layer has porous electrodes provided on respective opposite sides of the layer. By use of a sensor body which has the above-mentioned cells and in which the second measurement chamber communicates with the first measurement chamber via a second diffusion control layer, the NOx concentration of an object gas is detected.
Specifically, a pump-current control means causes the first oxygen pump cell to pump oxygen out from the first measurement chamber such that an output voltage of the oxygen-concentration detection cell becomes constant, thereby controlling the object gas flowing into the second measurement chamber from the first measurement chamber to have a constant oxygen concentration. A constant-voltage application means applies a constant voltage to the second oxygen pump cell in such a direction as to pump out oxygen from the second measurement chamber. On the basis of current which flows through the second oxygen pump cell as a result of application of the constant voltage, a nitrogen-oxide-concentration detection means detects the NOx concentration of the object gas.
In order to accurately detect the NOx concentration by the above-mentioned method, the conventional nitrogen-oxide-concentration detection apparatus has a heater for heating the cells, since the gas sensor element must be heated to a predetermined activation temperature (e.g., 800° C. or higher) for activating the cells. The temperature of the gas sensor element is controlled as follows. The internal resistance of the oxygen-concentration detection cell is detected. With reference to a sensor temperature obtained on the basis of the detected internal resistance, energization of the heater is controlled. In another conventional gas sensor element, after the heater is energized, energization of the heater is controlled in such a manner that the oxygen-concentration detection cell assumes a target internal resistance.
However, the above-mentioned conventional gas sensor element tends to consume much time (herein, the time is called the activation time) until a sensor output (current flowing between the electrodes of the second oxygen pump cell) used to detect concentration of a specific gas (concentration of NOx gas) stabilizes to a value corresponding to a target concentration range (a “target concentration range of measurement”). Thus, a relatively long waiting time after startup of the gas sensor element must be set before the stabilization.